The objective of this study is to evaluate the effects of low levels of SO2 and respirable particulates on the health of human beings and to identify whether there is a degree of interaction between these pollutants at these levels. Sulfates and sulfites and nitrates and nitrites as well as ozone and nitrogen dioxide will also be monitored. Six communities have been selected that have different levels of SO2 and respirable particulates, two high, two intermediate and two low. Random samples of the adult population (25-74) have been selected and studied by means of a standard respiratory questionnaire and simple tests of pulmonary function. Occupational and residential histories have been obtained as well as smoking habits. A cohort of children from each community has been selected and studied annually. Both of these population groups are being followed prospectively. Historical data on the levels of air pollution helped us to select the communities. We have continued monitoring in each area by means of Central stations where continuous monitors are used and we have established 10 or more satellite stations in each community where we measure 24-hour samples for SO2, NO2 and mass respirable particulates (MRP) both indoors and outdoors. We have done some preliminary studies with personal samplers for MRP. We need to do more of this type of sampling in order to have a better estimate of the individual's actual exposure as he moves about during the day. We expect from all this air monitoring to have a better estimate of the exposure of the population rather than use only the data from a Central site. Neutron activation has been used to try to identify the source of the particulates and to determine whether the composition indoors is similar to that out-of-doors or not.